1. Field of the Invention
The invention relates to a reading apparatus for reading a first type of digital information recorded on a first longitudinal recording medium in the form of information tracks recorded at an angle to the direction of travel of said recording medium, according to a first format in which said tracks have been recorded on said medium at a given angle to said direction of travel, at a first track pitch and have a first track width, said first type of information being readable by said apparatus at a given tape velocity during a normal mode of operation of said reading apparatus, whereby said first type of information can be read from said medium at a first bit rate, immediately adjacent tracks exhibiting mutually different azimuth angles, each track exhibiting either a first or a second azimuth angle, a pilot tone having one of at least two different frequencies being recorded in every second information track, the pilot tone frequency cycling through said at least two different frequencies, said reading apparatus comprising at least one read head adapted to read tracks having said first azimuth angle and at least one read head adapted to read tracks having said second azimuth angle.
2. Description of the Related Art
An apparatus of this type is disclosed in, for example, U.S. patent specification U.S. Pat. No. 4,597,023, which is incorporated herein by reference.
Such an apparatus may accurately track the recorded information tracks, by detecting a crosstalk component in the playback signal originating from a track that is read, this crosstalk signal being caused by the tracking signals recorded in the neighboring tracks. The crosstalk component is used to accurately position the reading head with respect to the track from which information is to be read.
The tapes which are used as the longitudinal recording medium in such an apparatus may be blank tapes on which a user (re)records material, or may be prerecorded software tapes.
Two exemplary types of tape which may be used are Particulate and Metal Evaporated (ME) tapes. Examples of particulate tapes are metal particle (MP) tape and barium ferrite tape.
The particulate type of tape has the advantage that it is cheaper than the metal evaporated type, but has the disadvantage that it performs more poorly in terms of parameters such as S/N ratio, etc. In attempting to use a cheaper tape to carry prerecorded software, this poorer performance leads to problems, as explained below.
In order to produce prerecorded software tapes, the following processes are possible :
"Fast Copy". In this process, a master tape and the tape to be recorded are placed in contact, and a copy of the information on the master tape is transferred to the copy tape by a thermomagnetic or by an anhysteretic process, for example. The copying is carried out at a tape speed higher than the nominal tape speed. PA1 Alternatively, the master tape is placed in a master playing apparatus, and a number of slave recording apparatuses are used to record the information on copy tapes. This process is carried out at the nominal tape speed.
A characteristic of both these processes is that the signal-to-noise ratio on the copy tapes is reduced by several dB.
The solution to this problem, which is adopted in the VHS system, is that a master tape which has a higher signal level than the normally used tapes is chosen for use as a master tape. In the absence of such a master tape, the recording format must be changed. In the RDAT system, for example, the track width is greater than the head width, which results in an increased signal-to-noise ratio, thus compensating for the signal loss resulting from the high speed duplication process.
The disadvantage associated with the RDAT solution is that the linear tape speed is increased to make the track pitch greater, in order to accommodate the wider tracks. This causes an increase in the quantity of tape required, making the prerecorded software more expensive.
The use of the higher performance ME type of tape to carry prerecorded software (e.g. in the case of video tape: films, either for sale or for hire from a video library) results in increased duplication costs, as this type of tape cannot be used in the "Fast Copy" process.